In general, an image sensor is a semiconductor device for converting optical images into electrical signals, and is mainly classified as a charge coupled device (CCD) or a Complementary Metal Oxide Silicon (CMOS) image sensor.
A CMOS image sensor sequentially detects electrical signals of pixels in a switching scheme by forming photodiodes and MOS transistors in unit pixels.
A CMOS image sensor is simpler in a driving scheme than compared to a CCD image sensor. It is also capable of implementing a variety of scanning schemes and integrates signal processing in a single chip so that miniaturizing a product is possible. A CMOS image sensor uses CMOS technology so that manufacturing costs and power consumption are low.
A CMOS image sensor according to the related art has a substrate divided into a photodiode region for receiving a light signal to convert it into an electrical signal and a transistor region for processing the electrical signal.
A typical CMOS image sensor has a structure such that the photodiode and the transistor are horizontally arranged.
While the horizontal-type CMOS image sensor improves upon some of the disadvantages of the CCD image sensor, problems still exist in the horizontal-type image sensor.
According to the horizontal-type image sensor, the photodiode and the transistor are formed to be horizontally adjacent to each other on a substrate. Thus, the photodiode cannot encompass the entire area of a pixel region of the image sensor, leading to a reduced fill factor and/or limited resolution.
In addition, with a horizontal-type image sensor, it is very difficult to achieve optimization for the process of simultaneously manufacturing the photodiode and the transistor. A shallow junction is required for a low sheet resistance in a rapid transistor process, but the shallow junction may not be appropriate for the photodiode.
Furthermore, according to the horizontal-type CMOS image sensor, additional on-chip functions are added to the image sensor so that the sizes of unit pixels are increased or reduced in order to maintain the sensitivity of the image sensor.
When the size of the unit pixel is increased, the resolution of the horizontal-type CMOS image sensor is decreased. In addition, when the area of the photodiode is decreased, the sensitivity of the horizontal-type CMOS image sensor is decreased. Thus, there exists a need in the art for a CMOS image sensor and fabricating method with an improved fill factor.